The furnace industry, in its efforts to maximize utilization of fuel resources, have developed furnaces that have a thermal efficiency on the order of 90% and better. As the efficiency of the furnaces is increased there is a corresponding increase in the amount of liquid or moisture entrained in the exhaust gas which condenses out of the exhaust gas stream. Thus, the more efficient the furnace becomes, the more condensate is created. The amount of condensate being created is also dependent upon the relative humidity of the air being used for combustion.
The condensate in the exhaust, particularly in the housing of the draft inducer, is removed because it can cause performance and/or furnace operational concerns. For example, accumulating condensate in the housing can lead to a reduction in the air performance of the air blower which can cause the furnace to shutdown. Also since the condensate from exhaust gases is acidic there are furnace material selection concerns.
To deal with accumulating condensate, the housing of a furnace draft inducer has been provided with a drain connection to drain away the condensate accumulating within the housing. This drain connection, however, has been found to allow drainage of the condensate only when the blower of the draft inducer is in the de-energized condition. When the blower is energized (i.e., in operation), the suctional force developed by the venturi effect of the air flowing across the drain connection opening was sufficient to discourage or prevent the liquid condensate from exiting the condensate drain tube. Essentially, the suctional force at the drain opening establishes a negative pressure at the exit of the drain tube. In addition, although the centrifugal force of the rotating air stream tends to impinge the condensate against the housing scroll shell, the velocity of the air tries to swirl the liquid condensate past the drain opening that is typically flush with the shell surface. Thus because of the swirling fluid and the negative air pressure, drainage of the condensate from the housing was difficult to accomplish when the blower was in operation.
The inability of the condensate to be drained through the drain connection while the blower was running caused the furnace to shutdown the combustion process particularly with furnaces or units that created more than the usual amounts of condensate (e.g., high relative humidity). The furnace would shutdown because, as noted above, the buildup of condensate in the housing during the running mode reduced air performance of the blower. This reduction in performance was interpreted by the furnace's protective circuitry and logic to be a blocked flue condition.
For a blocked flue condition, the furnace draft inducer or blower is kept running to clear or eliminate the blocked flue. Since the blower is kept running, the condensate accumulating in the housing does not drain and thus the furnace remains in the shutdown condition (i.e., furnace cannot refire) because the reduction in performance conditions are still present. Typically, a service person had to be dispatched to diagnosis the problem and resolve it so the furnace could be restarted.
There is, therefore, a need for a means to drain the condensate from the exhaust of a furnace that is accumulating in the housing of an operating impeller and more particularly for draining condensate when using combustion air having a wide range of relative humidity.